The invention relates to a resonant circuit arrangement for anode resonant circuits in transmitter-output amplifiers of high frequency and high radio-frequency power output with a coaxial triode or tetrode as the transmitter tube.
The resonant circuit arrangement according to the invention is particularly advantageous in the frequency range of approximately 50 to 200 MHz and for a power output of 200 kW to approximately 2 MW, where the design of anode resonant circuits has hitherto encountered numerous technical problems.
Thus, one difficulty was that very large transmitter tubes were required for the aforementioned transmitter outputs and for the aforementioned frequencies even had to be structures which can resonate in the form of coaxial line portions coming close to the quarter-wave length resonance. Thus, e.g. a transmitter tube able to supply a 2 MW radio-frequency power at an operating frequency of approximately 100 MHz has a short-circuit resonance (quarterwave length resonance) at a frequency of 105 MHz. To obtain an effective energy disconnection at this frequency it is necessary to have at least a .lambda./2 structure as the external resonant circuit.
A further problem has involved the isolating capacitor in the anode circuit where, in the case of the parallel feed-in of the d.c. input power used here the feed-in point is at earth potential. Due to the fact that there are d.c. voltages of up to 25 kV at the capacitor and also radio-frequency currents can flow through it, it is exposed to considerable loads and is consequently a particularly critical components.
An attempt could be made to solve the aforementioned problems in that the line portion formed by the coaxial arrangement of the cylindrical anode and the cylindrical grid electrode of the tube is extended by a corresponding, externally fitted, coaxial conductor arrangement in such a way that the overall arrangement formed by the transmitter tube and the extension piece forms a .lambda./2 line portion. If the cylindrical anode member and its extension is insulated from the remainder of the structure, it can be exposed to the full anode d.c. voltage, which is however, superimposed on the radio-frequency a.c. voltage of the anode circuit. Due to the high voltage formed between the inner and outer conductors of this .lambda./2 line formed from the transmitter tube and the coaxial extension, it is necessary to make certain requirements regarding the distance between the inner and outer conductors as concerns the minimum diameter d of the inner conductor and the diameter D of the outer conductor. If possible and in view of the dielectric strength of the arrangement, a ratio of D/d.apprxeq.e=2.718 is preferably chosen. However, this leads, in accordance with the relationship Z.sub.o =13810 g.sub.10 (D/d), to a correspondingly high characteristic impedance Z.sub.o of the coaxial line system and consequently to disadvantageous matching conditions between transmitter tube and anode resonant circuit.
The characteristic impedance of the line portion formed by the tube is very low, because there can be a very small distance between the inner conductor (grid) and outer conductor (anode) due to the good insulation by the vacuum of the tube. A typical value is in this case 15 ohms. Under the conditions of air insulation, it would not be possible to extend the coaxial system for the same characteristic impedance, due to the high voltages present. On increasing the clearance, the characteristic impedance rises. However, the line current in the current loop, i.e. at the transition between the tube and the circuit, is determined by the a.c. voltage of the tube and the characteristic impedance of the tube system. Thus, this current also flows into the external system, where it causes in the voltage maximum a voltage as a product of the current and the characteristic impedance. This leads to a considerable transformation of the tube a.c. voltage to very high values.
As transmitter tubes of the aforementioned output powers are operated in water-cooled manner, particularly up to the boiling point, i.e. accompanied by steam formation, difficulties are also encountered in the arrangement of the cooling water pipes with respect to the inlet or output connections of the anode member constructed as a cavity for the head exchange and in which the coolant circulates. The limits for the dielectric strength of a system designed for these conditions are rapidly reached, so that preference is given to the insertion of an isolating capacitor between the anode and the inner conductor in order to keep the latter free from d.c. current. However, it is then necessary to use an especially designed isolating conductor with a particularly high-grade dielectric, because the entire resonant circuit current flows through the latter and this can reach several thousand amperes corresponding to the transmitter output and the resonant circuit quality.
In order to obtain improved matching conditions, it is also possible to extend the anode resonant formed from the coaxial conductor arrangement of transmitter tube and extension piece to 1=3.lambda./4 (.lambda. being the operating wavelength). However, in this case it is not possible to obviate the use of the isolating capacitor.
Another way to form an anode resonant circuit is, for example, to design a cylindrical cavity resonator for the resonance with the E.sub.010 wave mode. According to Meinke-Gundlach "Taschenbuch der Hochfrequenztechnik", 2nd edition, section G.7 such a cavity resonator is can-shaped, i.e. constructed as a flat can. If, for example, an operating frequency of 108 MHz is required, in the case of a .lambda./2 resonance of this cavity resonator, a diameter of 2.5 m is obtained for such a can, whose electrical length is extended by .lambda./4 by the coaxial line portion formed from the actual transmitter tube and is therefore increased to an electrical length of 3.lambda./4. Thus, such an arrangement has a considerable space requirement and has a tendency to undesired interfering modes.